Interior Minister Arsen Avakov's evaluation appeared on Facebook Saturday, shortly after armed militants with Russian weapons seized more government buildings in the Russian-speaking east, including police headquarters in Donetsk and Kramatorsk.

Witnesses, including western journalists, say the Kramatorsk facility was captured after a firefight, but there were no immediate reports of casualties.

Armed men stand in front of police headquarters in Slaviansk, April 12, 2014.

The takeover of police facilities in Donetsk prompted the city's police chief to resign, while elsewhere, Western news accounts late Saturday said militants controlled the eastern city of Sloviansk.

Moscow has repeatedly denied any role in Ukraine's unrest, which erupted in full two months ago, when anti-Russian protesters in Kyiv forced then-president Viktor Yanukovych to flee the country.

US reaction

Meanwhile, the United States has called on Russia to "cease all efforts" to destabilize Ukraine. A White House National Security Council spokeswoman said Saturday the United States is concerned that Russian separatists -- with apparent support from Moscow -- are "inciting violence and sabotage" against the Ukrainian state.

Secretary of State John Kerry spoke by telephone with Russian Foreign Minister Sergei Lavrov, the State Department said.

"During the call Kerry expressed strong concern that attacks today by armed militants in eastern Ukraine were orchestrated and synchronized, similar to previous attacks in eastern Ukraine and Crimea,'' said a senior State Department official, speaking on condition of anonymity.

"Militants were equipped with specialized Russian weapons and the same uniforms as those worn by the Russian forces that invaded Crimea. The secretary made clear that if Russia did not take steps to de-escalate in eastern Ukraine and move its troops back from Ukraine's border, there would be additional consequences,'' the official added.

The official did not state what those consequences would be.

US officials met Friday with Ukrainian finance officials to discuss a "range of strategic and economic issues" according to a State Department statement.

In Moscow, Russian state television reports Foreign Minister Sergei Lavrov told U.S. Secretary of State John Kerry Saturday that the crisis is caused by the Kyiv government ignoring "the legitimate needs and interests of the Russian and Russian-speaking population" of the region.

Lavrov also warned that any use of force by Kyiv could undermine diplomatic efforts to resolve the crisis.

Donetsk

Members of the disbanded Ukrainian riot police unit Berkut arrive to support pro-Russian protesters in Donetsk, Apr. 12, 2014.

Around 200 pro-Russian protesters armed with clubs and knives took over the building Saturday and met no resistance -- forcing the police chief to resign.

“Here in Donetsk, everyone is sick of the powers in Kyiv,” complained Oleksander Korfman, one of the organizers. “That includes the police. They can’t break their pledge to the state, but they are with the people.”

Later around a dozen heavily armed Ukrainian police -- known as Berkut -- were welcomed into the headquarters. It appears there is much sympathy for the protesters among security forces.

Most of the protesters then left the police building in the hands of special forces. One protester -- who did not want to be named -- said the takeover was aimed at ensuring police weapons are not used against the demonstrators.

“The special forces will secure the place so no one steals the weapons inside. We will go back to our positions at the regional administration building," the protester said.

Armed pro-Russian groups also stormed police buildings in the nearby city of Slovyansk Saturday. There were reports that the demonstrators seized around 400 firearms. One who gave the name Sergei explained their actions.

“Our people want to live quietly and peacefully, without the junta who seized power in Kyiv," he said, "and so that we are not under America and the West. We don't want to be their slaves. We want to be with Russia.”

Building by building, the protesters are gaining ground. Their challenge to Kyiv's authority is so far unanswered; the dangers of conflict are growing.

An image released by NATO on April 10, 2014 that shows fighter jets and helicopters at Primorko-Akhtarsk Air Base, Russia, near Ukraine. (DigitalGlobe/NATO ACO PAO)

NATO says Russian armed forces are massing on Ukraine's eastern border, while Moscow says they are on normal maneuvers.

Ukraine suspends gas payments

There were other signs of heightened cross-border tensions Saturday, with the Kyiv government saying it is suspending natural gas payments to Moscow. Details of the move were not immediately clear.

Moscow says its neighbor owes $ 2.2 billion in payment arrears. Early this month, the Russian energy giant Gazprom announced two price increases that effectively raise Ukrainian gas costs by about 80 percent. Additionally, Russian President Vladimir Putin has hinted that Moscow may begin demanding energy payments from Kyiv at the time of delivery.

Top diplomats from Russia, the United States, Ukraine and the European Union are set to hold emergency talks on the crisis April 17 in Geneva.